Stroboscopic spark advance measuring apparatus

ABSTRACT

Apparatus for determining the spark advance of an internal combustion engine by the use of a stroboscopic lamp in which a sawtooth voltage is initiated with the occurrence of a regularly repeating electrical impulse such as the firing of a particular spark plug and in which the stroboscopic lamp is energized during each cycle of the sawtooth voltage when the instantaneous amplitude of the sawtooth voltage reaches a predetermined value, the selected value of the instantaneous amplitude being determined by an adjustable means which selects a portion of the sawtooth voltage and applies it to a voltage comparison circuit which in turn controls the conductivity of the lamp. The adjusting means is calibrated in degrees of the cycle of the operation of the engine. The amplitude of the sawtooth voltage is determined by a capacitor charging circuit and the timing rate of the capacitor charging circuit is adjusted in accordance with its maximum amplitude to maintain the same constant.

United States Patent [72] Inventors Richard S. MaC Crea New Brighton;

, Joseph A Marine, Hopkins, both of, Minn. [21] Appl. No. 771,133 [22]Filed Oct. 28, 1968 [45] Patented Aug. 3, 1971 [73] Assignee MarquetteCorporation [54] STROBOSCOPIC SPARK ADVANCE MEASURING APPARATUS 7Claims, 1 Drawing Fig.

[51] Int. Cl F02p 17/00 [50] Field of Search 324/15- [56] ReferencesCited UNITED STATES PATENTS 2,715,711 8/1955 Wells 324/16 2,785,2153/1957 Yetter 324/16 3,368,143 2/1968 Roberts... 324/16 3,499,322 3/1970Pelta 324/16 X Attarneys- Frederick E. Lange, John J. Held, Jr. andEugene L. Johnson ABSTRACT: Apparatus for determining the spark advanceof an internal combustion engine by the use of a stroboscopic lamp inwhich a sawtooth voltage is initiated with the occurrence of a regularlyrepeating electrical impulse such as the firing of a particular sparkplug and in which the stroboscopic lamp is energized during each cycleof the sawtooth voltage when the instantaneous amplitude of the sawtoothvoltage reaches a predetermined value, the selected value of theinstantaneous amplitude being determined by an adjustable means whichselects a portion of the sawtooth voltage and applies it to a voltagecomparison circuit which in turn controls the conductivity of the lamp.The adjusting means is calibrated in degrees of the cycle of theoperation of the engine. The amplitude of the sawtooth voltage isdetermined by a capacitor charging circuit and the timing rate of thecapacitor charging circuit is adjusted in accordance with its maximumamplitude to maintain the same constant.

112 -2OV 120 am 7 0 A88 114 117 STROBOSCOPIC SPARK ADVANCE MEASAPPARATUS I BACKGROUND OF THE INVENTION URING It is quite common in theengine diagnostic art to employ a stroboscopic lamp for illuminating aselected part of an internal combustion engine and adjustably delayingthe energization of the lamp until the selected part of the internalcombustion engine appears to be at the desired position. The

amount of adjustment is then observed and acts to indicate the extent ofengine ignition advance. A typical system of this type is shown in theWells et al. US. Pat. No. 2,7l5,7l l.

While prior devices of this type have provided a convenient means ofobserving theamount of ignition advance, they have suffered from thedrawback that the means for adjusting the delay in energization of thestroboscopic lamp is independent 1 of the operation of the engine andhence the ratio of the amount of delay to the length of the engine cycleis affected by any variation in speed of the engine. Thus, with priordevices of this type, it has been necessary to run the engine at apredetermined selected speed and to maintain this speed by observing atachometer or other speed-measuring device.

SUMMARY OF THE INVENTION selected value. Adjusting means is provided forselecting the amplitude at which the light is tired and this adjustingmeans may be provided with indicating means calibrated in degrees ofthecycle ofoperation of the engine.

The apparatus further contemplates means for maintaining the maximumamplitude of the sawtooth voltage constant regardless of engine speed.Such sawtooth voltages are commonly generatedby the use of acapacitor-charging circuit. This would tend to result in a loweramplitude as the engine speeds up since the capacitor does not have asmuch time to charge during the cycle of the enginet ,:Tli'e presentinvention.

contemplates means for adjusting the timing rate of thecapacitor-charging circuit so as to maintain the maximum amplitude ofthe sawtooth voltage substantially constant re-.

gardless of the speed at which the engine is being operated.

This is done by means responsive to the maximum amplitude:

of the sawtooth voltage which adjusts the conductivity of an electronicimpedance in series with the capacitor in the charging circuit inaccordance with the amplitude of the sawtooth voltage.

The adjustable means for selecting the instantaneous am-: plitude of thesawtooth voltage at which the lamp is fired con-1 sists of means forselecting a variable portion of the sawtooth voltage and applying thisvariable portion to a voltage com-- BRIEF DESCRIPTION OF THE DRAWING Thesingle FIGURE of the drawing shows the apparatus in schematic form inconnection with the spark plugs, ignition system and flywheel ofaconventional internal combustion engine.

DESCRIFTION OF THE PREFERRED EMBODIMENT Referring to the drawing, thepresent invention is shown in connection with an internal combustionengine 10 shown in highly schematic form. Extending from the engine is acrankshaft 11 to which is secured a pulley 12 having a flange 20. At asuitable point on the flange 20 of pulley 12 a timing mark 13 appears.In adjusting the ignition, it is desirable to adjust the ignition withrespect to a predetermined position of the crankshaft. The timing markis employed to determine when the crankshaft is at a predeterminedposition. By stroboscopically illuminating the flywheel at a frequencybased upon the speed of engine rotation, the timing mark appears tostand still. By adjustably delaying or advancing the point at which thestroboscopic light is illuminated during each cycle of the engine, theapparent position of the timing mark can be adjusted. By adjusting thestroboscopic light in this manner and observing the amount of theadjustment necessary to bring the timing mark to the desired position,it is possible to determine the extent to whichthe ignition is advancedor retarded with respect to the desired amount of advance.

Referring again to the drawing, I have shown a conventional timing light14 comprising a housing 15 in which is located a gas-filled flash tube16 having a trigger electrode 17. The light from the flash tube isdirected through a tubular light guide 18. The housing 15 is alsopreferably provided with a handle 19 so that the unit may be held by themechanic in such a manner that the light from the flash tube 16 is sodirected onto the pulley-20 as toilluminate the timing mark 13.

The timing light 14 normally includes certain electrical elementswithinthe housing 15 and which are associated with the flash tube 16 tocause the same to fireat the desired time. For convenience inillustration, these various electrical elements are shown as locatedoutside of the housing 15.

These elements associated with the timing light and normally included inthe housing 15 will now be described. The main terminals of the flashtube 16 are connected across a capacitor 26 of larger capacity. Capacity26 is connected between a source of positive voltage 21 and ground. Thesource 21 of positive voltage may, for example, be a source of voltageat +500 volts. The capacitor 26 is connected between this source ofvoltage 21 and ground through a circuit including a resistor 22,conductor 23, capacitor 26 and conductor 24 to ground at 25. During theperiods when'the tube 16 is nonconductive, this capacitor 26 is chargedup. When the lamp 16 is rendered conductive by the application of avoltage to the trigger electrode 17, thecapacitor 26 is rapidlydischarged through the tube 16 to produce a high intensity flash of veryshort duration.

The voltage for the trigger electrode 17 is produced by a triggertransformer 28 having a low voltage primary 29 and a high voltagesecondary 30. One terminal of the secondary winding 30 is connected toground and the other terminal is connected by conductor 32 to thetrigger electrode 17. The energization of the primary winding 29 iscontrolled by a silicon-controlled rectifier 33. The primary winding 29is connected in a circuit extending from a terminal 35 connected to apositive source of voltage such as +50 volts, through a resistor 36, acapacitor 37, primary winding 29 and conductor 24 to ground. Thesilicon-controlled rectifier, or SCR, 33 has its anode connected to thejunction of resistor 36 and capacitor 37 and has its cathode connectedto ground. When the SCR 33 is nonconductive, current flows through thecapacitor 37 and primary winding .29. until the capacitor 37 is fullycharged. 'Due to the relatively high resistance value of resistor 36,the increase in the current through winding 29 occurs at a relativelyslow rate, and very little voltage is induced in the secondarywinding30. As soon, however, as the SCR 33 is rendered conductive, thecapacitor 37 is discharged rapidly through the SCR 33 and theprimarywinding 29. This causes a rapid change in the current through winding 29to result in a high voltage being generated'in secondary winding 30which voltage is applied to the trigger electrode 17 of the flash tube16 to fire the same. The firing of the SCR 33 is caused by theapplication ofa firing voltage to the firing electrode 39 of the SCR 33.Normally, the firing electrode 39 is maintained at substantially thepotential of the cathode by a connection between the firing electrode 39and ground through a resistor 40. When it is desired for the SCR tobecome conductive, a positive voltage is applied to the firing electrode39 as will later be described.

Referring again to the engine 10, the engine is illustrated as being asix-cylinder engine. It is of course, understood that the invention isequally applicable to a four or eight-cylinder engine. The engine isprovided with six spark plugs 42-47. Each spark plug is connected to adifferent terminal of a distributor 48 of conventional construction.This distributor comprises a distributor arm 49 which is driven by theengine and which sequentially makes contact with a plurality ofterminals 50, each of which is connected to a different one ofthe sparkplugs 42-47. The terminals 50 are shown as connected to the variousplugs in such a manner as to fire these in the desired firing order fora six-cylinder engine.

The voltage for firing the spark plugs 4247 is produced by an ignitiontransformer 52 having a low voltage primary winding 53 and a highvoltage secondary winding 54. The low voltage primary winding 53 isconnected to the positive terminal of an automobile battery 56 through aconventional ignition switch 57. The opposite terminal of battery 56 isconnected to ground at 58. The lower terminal of primary winding 53 isconnected to a switch blade 59 which cooperates with a second switchblade 60 to form the conventional distributor breaker points. Switchblade 60 is grounded at 61. Cooperating with switch blade 59 is asix-sided cam 62 which serves to separate switch blades 59 and 60 sixtimes for each revolution of cam 62. It is, of course, understood thatif an eight-cylinder engine were being employed, the cam 62 would beeight sided. The usual capacitor 63 is connected across the points 59and 60. Each time that the points 59 and 60 are open, there is an abruptchange in the current flowing through the primary winding 53 so as toinduce a relatively high voltage in the secondary winding 54. Thisvoltage is sequentially applied to the various plugs by the distributor48 in the conventional manner.

Referring now to the apparatus for controlling the SCR 33 and hence thefiring of the stroboscopic light 16, this comprises a plurality oftransistors 66 and 67 which control the charging of a capacitor 68. Thecapacitor is periodically discharged by a further transistor 69 which,as will be presently described, is rendered conductive each time that afiring voltage is applied to the plug 47. Transistor 66 is an NPNtransistor and transistor 67 a PNP transistor. the two transistors beingconnected in series with each other and with the capacitor 68 between apositive bus conductor 71 and a negative bus conductor 72. The positivebus conductor 71 is connected to a positive source of voltage indicatedby the terminal 70 and which may, for example, be at +l0 volts. Thenegative bus conductor 72 is connected to a terminal 73 ofa negativesource of voltage such as -l0 volts. Also connected between the positivebus conductor 71 and negative bus conductor 72 are a pair of resistors75 and 76, the resistor 77 ofa rheostat 78, and a resistor 79. The baseoftransistor 67 is con nected to the junction of resistors 75 and 76 soas to be maintained at a constant potential. Transistors 66 and 67 actas a variable impedance in series with capacitor 68 to control thecharging rate of capacitor 68. The conductivity of transistor 66 andhence transistor 67 is controlled by the voltage across a capacitor 8]connected between the base and collector of transistor 66. The voltageexisting across capacitor 81 is in turn controlled by two NPNtransistors 83 and 84 connected in a Darlington type ofcompound circuit.Transistor 84 is connected between the positive bus conductor 71 andground through a circuit including a resistor 85, a conductor 86,transistor 84 and a resistor 87 to a ground connection 88. lt will beobvious that as the conductivity of transistor 84 is increased, thevoltage drop across resistor will increase causing the lower end tobecome less positive with respect to the upper end. The resistor 85 hasconnected in parallel therewith a capacitor 91. The parallel resistor 85and capacitor 91 are in turn connected across capacitor 81 through aresistor 92. The base of transistor 83 is connected through conductors93 and 94 to the upper or positive terminal of capacitor 68.

As previously noted, transistor 69 is connected across capacitor 68 andis effective to periodically discharge the same. It will be noted thatthe emitter of transistor 69 is connected to the lower terminal ofcapacitor 68 while the collector is connected through a resistor 96 andconductor 94 to the upper terminal of capacitor 68. Thus, whentransistor 69 is conductive, the transistor 69 acts as a short circuitexcept for the impedance of the resistor 96 which has a relatively smallresistance value.

The conductivity of transistor 69 is controlled by a connection to theNo. l spark plug 47. The base of transistor 69 is normally connected tothe negative bus conductor 72 through a resistor 98 so as to be normallymaintained at the same potential as the emitter to prevent conductionofthe transistor 69. The base is also connected, however, through acapacitor 99, a wave shaping circuit 100 and a conductor 101 to thesecondary winding ofa coupling transformer 102, the primary ofwhich isconnected between the spark plug lead 103 and the No. I plug 47. [t isto be understood that the coupling transformer 102 is normally embodiedin a coupling cable which is connected between the spark plug lead andthe spark plug. The wave shaping circuit 100 may be ofany conventionaltype such as a square wave generator which converts the oscillatoryvoltage appearing across the secondary of coupling transformer 102 intoa square wave voltage. The capacitor 99 acts to differentiate thisvoltage resulting in positive and negative peaks. Rectifying diode 104connected in parallel with resistor 98 serves to prevent the negativepeaks from being applied to the base oftransistor 69 so that only thepositive peaks are applied. Thus, each time that a firing voltage isapplied to plug 47, a positive voltage is momentarily applied to thebase of transistor 69 to render the transistor conductive. This causescapacitor 68 to rapidly discharge. Immediately thereafter, the capacitorstarts to again charge through transistors 66 and 67 at a ratedetermined by the conductivity of these transistors which in turn iscontrolled by the voltage across capacitor 81. The result is that asawtooth voltage appears across the terminals of capacitor 68, thissawtooth voltage being in turn applied to the base oftransistor 83.

When the maximum value of the sawtooth voltage is at the desired value,the conductivity of transistors 83 and 84 will be such that the voltagedrop across resistor 85 and hence across capacitor 81 maintainstransistors 66 and 67 at a conductivity such that the capacitor reachesthe desired value at the end of the engine cycle. If the voltage acrosscapacitor 68 tends to rise due to the engine slowing down and henceproviding a longer engine cycle within which the capacitor 68 can becharged, the voltage applied to the base of transistor 83 will increaseto increase the conductivity of that transistor and hence of transistor84. This, in turn, causes a greater voltage drop to appear acrossresistor 85 and hence across capacitor 81. This in turn will tend tolower the potential of the base of transistor 66 with respect t1 theemitter causing the effective impedance of transistors 66 and 67 toincrease. This reduces the time constant of the circuit includingtransistors 66 and 67 and capacitor 68 so that a longer time is requiredfor the voltage across the capacitor 68 to reach the desired value. Itwill be obvious that if the engine speeds up so as to decrease thecharging time, transistors 83 and 84 will become less conductive to inturn reduce the voltage across capacitor 81 and to in crease theconductivity of transistors 66 and 67. This will in turn result in afaster charging rate for capacitor 68. With this arrangement, it will beapparent that the charging rate of capacitor 68 is automaticallyregulated to maintain the maximum voltage across it at a relativelyconstant value.

It will be obvious from the above that the apparatus, as just described,produces a sawtooth voltage each cycle of which begins at the time thatthe No. I plug 47 is first fired and continues to increase steadily inamplitude until the plug again 14 to fire when the instantaneous valueof the sawtooth voltage approaches a predetermined magnitude.

To accomplish the purpose just discussed, we utilize a voltagecomparison circuit consisting of two NPN transistors 107 and 108.Transistor 107 has its collector and emitter connected in a circuitextending from the positive bus conductor 71 through a resistor 110, thecollector and emitter of transistor 107 and a resistor 111 to a negativevoltage source indicated by the terminal 112 which may, for example, beat volts. The collector and emitter of the other transistor 108 areconnected between the positive bus conductor 71'and the upper terminalof resistor 111 so that the-collector and emitter of transistor 108 areconnected in parallel with the re sistor and the collector-emittercircuit of transistor 107. The base of transistor 107 is connectedthrough conductors 114 and 94 to the upper terminal of capacitor 68 sothat the voltage applied to the base of transistor 107 is alwaysdependent upon the instantaneous magnitude of the sawtooth voltage. Thebase of transistor 108 is connected by conductor 114 to the slider ofpotentiometer 78. Since the resistor 77 of potentiometer 78 is connectedin series with a plurality of fixed resistors between the positive busconductor 71 and the negative bus conductor 72, the potential at theslider ofpotentiometer 78 remains constant for any given position of theslider. The

,base of transistor 108 is thus maintained at a constant value dependingupon the position of the slider. Whenever the base potential oftransistor 107 exceeds that of transistor 108, current will flow throughtransistor 107 to cause a voltage drop across resistor 110. The point atwhich this occurs in the cycle of the sawtooth voltage will depend uponthe setting of the slider of potentiometer 78. When transistor 107 doesbecome conductive, the lower terminal of resistor 110 becomes negativewith respect to the upper terminal. The resistor 110 is connectedbetween the base and emitter of a further PNP transistor 116. Theemitter and collector of transistor 116 are connected between thepositive bus conductor 71 and the negative bus conductor 72 in serieswith a resistor-117.

When the transistor 107 becomes conductive to produce a voltage dropacross resistor 110, this results in the base of transistor 116 becomingnegative with respect to the emitter to cause transistor 116 to becomeconductive to in turn produce a voltage drop across resistor 117, thepolarity of this voltage drop being such that the upper terminalofresistor 117 is positive with respect to the lower terminal. The upperter minal of resistor 117 is in turn connected through a conductor 119,a capacitor 120 and a conductor 121 to the gating electrode of SCR 33.The capacitor 120 serves to differentiate the voltage across resistor117 causing a sharp positive peak to be. applied to the gating electrode39 whenever the transistor 116 becomes conductive. This in turn causesSCR 33 to become conductive to produce an abrupt change in currentthrough primary winding 29 of trigger transformer 28. This causes avoltage to be applied to the trigger electrode 17 of gas filled tube 16.in the manner previously described. Thus, whenever the instantaneousvalue of the sawtooth voltage approaches a predetermined value dependentupon the setting of the slider of potentiometer 78, the flash tube 16 iscaused to fire to illuminate the timing mark 13. The point in the cycleof the sawtooth voltage at which this occurs is determined by thesetting of the slider of potentiometer 78. The slider of potentiometer78 is operatively connected to an adjusting knob 125 which may becalibrated in degrees of advance ofthe ignition.

While we have shown the majority of the various circuit components asoutsideof the housing 15 of the timing light for convenience inillustration, it is to be understood that normally all of the apparatusdescribed above with the exception of the knob 125 and the engine beingtested will be located within the housing 15. in such an arrangement.the knob 125 will be mounted on the wall of the housing in such aposition that it can readily be adjusted by the mechanic using theequipment.

Referring now to the overall operation of the system, the apparatus isconnected to an engine by disconnecting the spark plug lead 103 andinserting between the lead and the plug 47 the primary of couplingtransformer 102. With the engine running and with the various powerconnections effective, a sawtooth voltage is generated, this sawtoothvoltage being started each time that a firing voltage is applied to plug47 and terminating when this firing voltage is again applied. Themaximum voltage amplitude of the sawtooth voltage is maintained constantby varying the conductivity of transistors 83 and 84 in accordance withthe maximum value of the sawtooth voltage. This in turn varies thevoltage across capacitor 81 to in turn vary the conductivity oftransistors 66 and 67 to affect the time constant of the chargingcircuit for capacitor 68. In this way, as the engine speeds up or slowsdown, the charging rate of capacitor 68 is adjusted accordingly tomaintain the maximum value of the sawtooth voltage always constant. Onceeach cycle, the flash tube 16 is energized. The flash tube 16 isenergized at a frequency which has a fixed relationship to the speed ofthe engine since the flash'tube is energized each time that the plug 47has a firing voltage applied thereto. if the operator holds the timinglight 14 so that the light from it illuminates the mark 13, the mark 13will appear to stand still. The operator can now adjust the knob 125 andthe mark 13 will apparently shift in position due to the fact that thepoint in the engine cycle at which the timing light flashes is shifted.All that the operator needs to do now is to adjust the knob 125 untilthe timing mark appears to be in the desired position. The position ofthe knob 125 can now be read and this will in dicate the number ofdegrees advance in the ignition timing. Because of the fact that thetime in the engine cycle at which the tube 16 is fired is dependent upona sawtooth voltage directly related to the operation of the engine, theadjustment of the'firing of the timing light will be related to theengine operation. By maintaining the amplitude of the sawtooth voltageconstant, the measurement of the spark advance is always constant and isnot affected in any way by engine speed.

It will thus be seen that we have devised a simple means for themeasurement of the amount of ignition advance which is independentofengine speed. Thus, it is no longer necessary to control accuratelythe engine speed during the measurement of the amount ofignitionadvance.

While we have shown a specific embodiment of our invention for purposesof illustration, it is to be understood that the scope of the inventionis limited solely by the appended claims.

We claim:

1. Apparatus for determining the spark advance of an internal combustionengine having a timing mark for indicating a particular position ofengine rotation, a spark plug, means for producing a high potentialfiring voltage for the spark plug including breaker points, and meansfor applying to the spark plug the firing voltage at a timesubstantially the same as that at which the points open, said apparatuscomprising:

a coupling circuit for producing discrete pulses at its outputcoincident with the application thereto of discrete input signals,

a connector for coupling the input of said coupling circuit to a sparkplug of the internal combustion engine whose spark advance is'to bedetermined,

means connected to said coupling circuit for generating a sawtoothvoltage each cycle of which is initiated as a result ofa discrete pulsefrom said coupling circuit resulting from the application to said plugof a firing voltage,

means for maintaining the maximum amplitude of the sawtooth voltage at aconstant value,

an electrically energized lamp for illuminating the timing mark oftheengine,

adjustable means for selecting a predetermined fixed value of voltage,comprising an adjustable impedance, a manually movable element connectedto said impedance for adjusting the same, and indicia means calibratedin degrees of the cycle of operation of the engine, a portion of theindicia means being positioned by said manually movable element so thatsaid indicia means always indicates the position of the manually movablemember in terms ofdegrees ofthe cycle of operating means, and

voltage comparison means for comparing said fixed value of voltage withthe instantaneous value of said sawtooth voltage and causingenergization of said lamp each time that the instantaneous value of saidsawtooth voltage bears a predetermined relation to said fixed value ofvoltage so that said lamp is illuminated once each cycle of saidsawtooth voltage at a time which differs from the time at which thefiring voltage is applied to the plug by the extent of adjustment ofsaid adjustable means so that the amount of the spark advance isindicated by said in dicating means associated with the adjustable meanswithout the use of any means for measuring the elapsed time between theapplication of the firing voltage to said plug and the timeofenergization ofsaid lamp.

2. The apparatus ofclaim 1 in which the means for generating a sawtoothvoltage comprises a capacitor charging circuit and in which there ismeans for adjusting the impedance in said capacitor charging circuit tovary the timing rate of said circuit and maintain the maximum amplitudeof said sawtooth voltage substantially constant regardless of the speedat which the engine is being operated.

3. The apparatus of claim 2 in which the means for adjusting the timingrate of said capacitor charging circuit comprises means responsive tothe maximum amplitude of said sawtooth voltage.

4. The apparatus of claim 2 in which the means for generating a sawtoothvoltage comprises a capacitor charging circuit including an electronicimpedance in series with a capacitor and in which there is meansresponsive to the maximum amplitude ofsaid sawtooth voltage for varyingthe conductivity of said electronic impedance.

5. The apparatus of claim 4 in which the electronic impedance is atransistor.

6. The apparatus ofclaim 1 in which the energization of the lamp iscontrolled by an electronic switch and in which the conductivity of saidelectronic switch is controlled by said voltage comparison means.

7. The apparatus of claim 1 in which the engine is a multicylinderengine having a spark plug for each cylinder with means for sequentiallyapplying a firing voltage to each ofsaid spark plugs and in which eachcycle of the sawtooth voltage is initiated at the time that a firingvoltage is applied to a selected one of said spark plugs.

1. Apparatus for determining the spark advance of an internal combustionengine having a timing mark for indicating a particular position ofengine rotation, a spark plug, means for producing a high potentialfiring voltage for the spark plug including breaker points, and meansfor applying to the spark plug the firing voltage at a timesubstantially the same as that at which the points open, said apparatuscomprising: a coupling circuit for producing discrete pulses at itsoutput coincident with the application thereto of discrete inputsignals, a connector for coupling the input of said coupling circuit toa spark plug of the internal combustion engine whose spark advance is tobe determined, means connected to said coupling circuit for generating asawtooth voltage each cycle of which is initiated as a result of adiscrete pulse from said coupling circuit resulting from the applicationto said plug of a firing voltage, means for maintaining the maximumamplitude of the sawtooth voltage at a constant value, an electricallyenergized lamp for illuminating the timing mark of the engine,adjustable means for selecting a predetermined fixed value of voltage,comprising an adjustable impedance, a manually movable element connectedto said impedance for adjusting the same, and indicia means calibratedin degrees of the cycle of operation of the engine, a portion of theindicia means being positioned by said manually movable element so thatsaid indicia means always indicates the position of the manually movablemember in terms of degrees of the cycle of operating means, and voltagecomparison means for comparing said fixed value of voltage with theinstantaneous value of said sawtooth voltage and causing energization ofsaid lamp each time that the instantaneous value of said sawtoothvoltage bears a predetermined relation to said fixed value of voltage sothat said lamp is illuminated once each cycle of said sawtooth voltageat a time which differs from the time at which the firing voltage isapplied to the plug by the extenT of adjustment of said adjustable meansso that the amount of the spark advance is indicated by said indicatingmeans associated with the adjustable means without the use of any meansfor measuring the elapsed time between the application of the firingvoltage to said plug and the time of energization of said lamp.
 2. Theapparatus of claim 1 in which the means for generating a sawtoothvoltage comprises a capacitor charging circuit and in which there ismeans for adjusting the impedance in said capacitor charging circuit tovary the timing rate of said circuit and maintain the maximum amplitudeof said sawtooth voltage substantially constant regardless of the speedat which the engine is being operated.
 3. The apparatus of claim 2 inwhich the means for adjusting the timing rate of said capacitor chargingcircuit comprises means responsive to the maximum amplitude of saidsawtooth voltage.
 4. The apparatus of claim 2 in which the means forgenerating a sawtooth voltage comprises a capacitor charging circuitincluding an electronic impedance in series with a capacitor and inwhich there is means responsive to the maximum amplitude of saidsawtooth voltage for varying the conductivity of said electronicimpedance.
 5. The apparatus of claim 4 in which the electronic impedanceis a transistor.
 6. The apparatus of claim 1 in which the energizationof the lamp is controlled by an electronic switch and in which theconductivity of said electronic switch is controlled by said voltagecomparison means.
 7. The apparatus of claim 1 in which the engine is amulticylinder engine having a spark plug for each cylinder with meansfor sequentially applying a firing voltage to each of said spark plugsand in which each cycle of the sawtooth voltage is initiated at the timethat a firing voltage is applied to a selected one of said spark plugs.